Two-dimensional reference objects for the inspection of coordinate measuring machines and machine tools in the form of ball and hole plates allow the errors of such machines to be measured with less effort (only one reference object is needed), thereby faster (typically in one day), and more cheaply (a reference object costs about $10000) as compared with other methods that require typically five days, require a set of costly reference measuring devices (typically $100,000 of worth in total), and are difficult to handle: usually, two different laser interferometers are needed, plus two electronic level meters, plus one straight edge, plus one granite square [1,2,3].
In spite of these obvious advantages, a break-through in the spread of such reference objects has not yet been reached because so far, commercially available ball plates were not optimised neither economically nor from the technical point of view. Better acceptance would be more likely if the reference objects were optimised insofar as they
are individually tailored to the dimensions of the machines to be inspected, PA1 have arrays of probing elements with a small raster spacing (between 20 mm and 50 mm), PA1 are of light weight so that they can be handled by one person (less than 20 kg), PA1 allow materials with low thermal expansion to be used, PA1 are not more expensive than the existing ball and hole plates which do not have all the properties above (about $10000 in 1996).
A main factor of the costs for a ball or hole plate of state-of-the-art plate design is the extensive fine machining required for the bulk material used. For example, the ball seats are milled with narrow tolerances to allow to thermally shrink the balls into their matching seats (holes). Plates from rolled steel additionally require annealing for dimensional stability and surface treatments for corrosion protection.
Reducing weight by applying bores in the plate material turned out to weaken the structure in certain directions (e.g. low stiffness in the diagonals reduced the accuracy with which squareness is represented). The milling of pockets leaving a thin wall in the plate's center plane and furnishing ribs for the stability against bending turned out to be satisfactory for smaller plates but significantly increased their price. Plates bigger than 0.8 m.times.0.8 m with holes or pockets turned out to bend excessively (more than 0.3 mm are not allowed to keep cosine errors negligible). No state-of-the-art design allowed plates bigger than 0.5 m.times.0.5 m to be handled by only one person because of their weight.